A method of measuring the spot size or bunch length of intense charged particle beams is proposed. The relation between the size (widths and length) of a charged particle beam and the beam's electric field forms the basis for a sub-micron beam size monitor. When the beam passes through a low pressure gas of high Z atoms, the beam field causes multiple ionizations of the gas atoms. The appearance of ionized atoms in a given charge state gives information about the field of the beam and hence its size. Sample calculations show that appearance thresholds can indicate the spot size of round beams with 10 nm accuracy or the bunch length of round or flat beams with up to 10 μm accuracy.
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